1. Field of the Invention
This invention relates to an optical amplification system suitably used for compensation of transmission loss and improvement of signal reception sensitivity by using an optical waveguide having a capability of optical amplification by optical pumping.
2. Prior Art
Optical amplification systems using optical fibers having an optical amplification capability have been known and utilized as means for switching optical transmission lines in optical CATV systems and other practical applications.
There has been reported that light of 1.55 .mu.m band can be effectively amplified by using a silicate glasses single mode optical fiber having an erbium (Er)-doped core in an optical amplification system of the above described type.
As illustrated in FIG. 9 of the accompanying drawings, an optical amplification system under consideration normally comprises a pumping source 31 for optical pumping, an optical combiner 32 for combining optical signals and pumped light and an optical fiber 33 having an rare earth element-doped core 33, to which an optical isolator 34 is added.
A known optical amplification system as shown in FIG. 9 normally shows a rise of excitation level in the optical fiber 33 when pumped light is introduced into the optical fiber 33 and amplifies light signals fed to the optical fiber 33 as it returns to a normal state from the raised excitation level.
Such an optial optical amplification system has a large gain and a high response speed and therefore is capable of adapting itself to an ultra-high speed transmission environment.
However, if no light signal is existent in the optical fiber 33 for a considerable period of time, e.g. several milliseconds, under a condition where pumped light is constantly being introduced to the optical fiber 33, the excitation level in the optical fiber 33 is raised further so that the light signal introduced in the optical fiber 33 under this condition is amplified by a large amplification factor accordingly.
Thus, if the optical fiber 33 remains under a condition where no optical signal is entered into it for a long period of time and immediately thereafter an optical signal is introduced there, the optical signal will be amplified by a very large amplification factor to generate intense optical pulses, which by turn can destruct and/or saturate the downstream systems.
FIGS. 10(A) and 10(B) of the accompanying drawings illustrate waveforms obtained when a known optical amplification system is used for a line switching system,
Of these illustrations, the waveform of FIG. 10(B), which is obtained before it is optically amplified, shows that the rising edge of the signal is amplified to generate strong pulses as may be more clearly seen when compared with that of FIG. 10(B) obtained after the optical amplification.
In view of the above described problem of the prior art, it is therefore an object of the present invention to provide an optical amplification system that can operate properly regardless of the waveform of the incoming signal.